A. Field of Invention
The present invention relates generally to devices for providing resistance to flow and more particularly to a new and improved fluid resistor for installation within a bore and economically providing resistance to flow of fluid through the bore.
B. Description of Related Art
Known devices used for providing resistance to the flow of a fluid media, in their simplest form, frequently control the flow by simply restricting the internal diameter of an orifice. Severe reduction of an orifice causes a fluid resistor to become subject to significant changes in resistance in response to changes in the viscosity of the fluid media, as well as to contamination and to cavitation. For these reasons, it has been found to be important to maximize the orifice size for a given resistance by increasing the ratio of the resistance offered by the device to the cross-sectional area of the orifice. One method of increasing the ratio of resistance to orifice size is to increase the exposure of the fluid media to the internal surface area of the flow passage or device, thereby increasing the friction experienced by the flow. Multiple parallel orifice resistors such as screens or filters, by increasing the surface area exposed to flow, provide resistance with a relatively large total flow passageway area, but the individual orifices are small, leaving such devices susceptible to blockage by contamination. Arranging multiple resistors in series allows each individual orifice to have a greater internal diameter than a single orifice restrictor but such arrangements can be bulky and costly. A fluid restrictor which overcomes some of the performance limitations that arise from using either a single orifice restrictor, or a multiple orifice arrangement is shown in U.S. Pat. No. 3,323,550, issued to Lee, wherein there is described a fluid resistor constructed of a series of plates aligned normal to the overall direction of the flow forming a tortuous flow path. In the Lee device the fluid media first enters at the center of a first chamber from which flow exits through a passageway that is perpendicular to the overall flow path and tangential to the first chamber and to a second spin chamber which the fluid media then enters. The fluid media exits the second spin chamber by a coaxial bore that has a smaller internal diameter than the second spin chamber. The spinning fluid is forced to move radially inward and as it does so, the rotational velocity is accelerated until the fluid exits through the coaxial bore into a deceleration chamber from which it exits by a tangential passageway. The tangential velocity of the fluid within the coaxial bore maximizes the effectiveness of the bore in resisting passage of the fluid. The tangential passageways into and from the second spin chamber and the deceleration chamber extend in opposing directions so that the spin of the fluid media must be dissipated before the fluid exits the deceleration chamber. The output of a deceleration chamber of a spin chamber/deceleration chamber pair may serve as the input into the spin chamber of a subsequent pair, and the number of pairs so connected may be varied to achieve the desired amount of fluid resistance. In the Lee device the coaxial bores are all drilled in a single disc that is normal to the axis of the resistor and therefore, the direction of flow through the bores is repeatedly reversed. The primary benefit of the Lee device is the ability to provide fluid resistance with a minimal reduction of internal diameter of the smallest orifice, thereby reducing the likelihood of blockage and allowing filtration by protective screens to be either eliminated or relaxed allowing larger passages through the filtering screens. It has been shown that increasing the interior diameter of the smallest orifice of a fluid resistor will substantially increase the average useful life of the flow resistor in most circumstances where clogging, or cavitation damage, is the primary cause of failure. The Lee device provides such a fluid resistor with increased diameter orifice in a form requiring manufacture by forming a series of disks which are subsequently bonded together to complete and seal the internal flow passage way. This method of manufacture is more costly than simpler although less efficient devices. The laminated discs are housed within a body that is sturdy enough to endure the rigors of installation. Since the repeated pairs of opposing spin and deceleration chambers are arranged in the plane normal to the axis of the general flow passage, the total diameter of the body and the installation bore is relatively large with respect to the amount of resistance provided by the device. Although the ratio of the resistance to the diameter of the orifice at its most narrow point in the Lee device is lower than that of simple resistors, the ratio of the overall bore diameter to the orifice is large due to the bulk of the resistor discs and body. In addition, the installation method of the Lee resistor requires the precise reaming of an installation bore and minor variations in tooling or reaming technique can produce clearances between the outside surface of the plug and the inside surface of the installation bore that are significant to the retention and sealing of the resistor.